Optical rangefinder instruments utilize either visible or invisible light for measuring the distance to a remote object. Over the years, these instruments have found their place in a host of applications, in particular in automotive and transportation applications where, for example, they are integrated in various types of active systems intended to assist vehicle drivers and to promote a higher safety on the road. Most optical rangefinders that range objects located beyond a few meters away operate according to the time-of-flight (TOF) principle, which relies on the finite propagation speed of light. The TOF principle comes in various forms, including pulsed TOF, amplitude-modulation TOF, and frequency-modulation TOF techniques. In the pulsed TOF technique, a light source enclosed within the rangefinder emits a train of light pulses of very short duration. A part of the optical energy carried by each pulse is reflected by the aimed object to return back to the collecting aperture of the optical receiver of the rangefinder. Knowing the velocity of light in the air, the distance that separates the aimed object from the rangefinder is inferred from the time taken by the light pulses to propagate up to the aimed object and then back to the rangefinder. This time delay is usually measured by an electronic counter combined with peak detection and threshold comparator circuitry. The development of optical rangefinders benefited from ongoing efforts fueled by a strong demand for consumer-grade, compact products available at low cost and intended for ranging objects distanced from up to a few hundreds meters. As a result, the basic design of these rangefinder instruments now revolves around compact assemblies that typically comprise a laser diode transmitter emitting laser pulses with a duration of 5 to 50 ns (nanoseconds) at a near-infrared wavelength, an optical receiver including either a photodiode (typically an Avalanche PhotoDiode (APD)), amplifiers, automatic gain control (AGC) and timing discriminators. Further details about laser ranging principles can be found in M-C. Amann et al., “Laser ranging: a critical review of usual techniques for distance measurement”, (Optical Engineering, Vol. 40, No. 1, pp. 10-19, 2001).
The light pulses emitted from typical laser rangefinders can propagate over long distances while maintaining a very small transverse beam size typically in one Field Of View (FOV). This high-directional character of laser beams is of great usefulness for performing angularly-resolved detection and ranging of objects when combined with an angular scan of the aiming direction of the rangefinder. The small beam size results from the distinctive nature of laser light, in particular its high spatial and temporal coherences along with the availability of laser sources capable of radiating single-longitudinal and single-transverse mode laser beams. These factors combine to allow optical collimation of the laser light pulses in the form of a beam of very low divergence (angular spread), the collimation being designed using simple, off-the-shelf optics. The emission of highly-directional laser beams from optical rangefinders finds its best use when the laser light reflected by the aimed objects is detected with an optical receiver that senses over a narrow FOV. The FOV of an optical receiver is given by the ratio of the size of the photosensitive surface of the photodetector integrated in the receiver and the focal length of the objective lens, the photosensitive surface being placed close to or exactly at the focal plane of the lens. In fact, optimum performances are generally obtained when matching the FOV of the optical receiver with the divergence angle of the emitted light pulses. In practice, the FOV of typical rangefinders does not exceed a few mrads (milliradians), mainly because of the quite small surface areas (typically in the range of 50 to 200 μm diameter) of commonly-available APDs along with the need for compact designs that command the use of objective lenses with focal lengths that do not exceed a few centimeters.
The APDs have become increasingly popular for integration in the optical receiver of laser rangefinders in such a way that rangefinders based on PIN photodiodes are now rarely encountered. Both PIN and APD photodiodes have sufficient bandwidth to detect optical pulse returns having durations in the ns range, and they can be made up of silicon for maximizing their quantum efficiency at near-infrared wavelengths lower than 1 μm. As compared to their PIN counterparts, APDs provide higher sensitivity and responsivity (up to 200×), owing to an avalanche multiplication process that enables the generation of multiple electron-hole pairs from the absorption of a single photon of light. In their analog (linear) regime of operation, APDs are reversely biased with a voltage slightly below their breakdown voltage to give an output photocurrent proportional to the light power that falls onto their photosensitive surface and to the gain applied. APDs then incorporate a gain mechanism internal to their structure, the gain factor being typically in the range of 50 to 200. However, one should note that the signal-to-noise ratio (SNR) of an APD first increases with the gain until reaching a peak value (optimum gain). The SNR then decreases with further gain due an excess noise factor intrinsic to the avalanche multiplication process. Although the higher detection sensitivity of APDs is fully exploited for optical detection in very low light level conditions, it is generally difficult to fully benefit from the advantages of APDs in rangefinder instruments intended for outdoor use in daytime conditions, for example in presence of bright sunlight. In these conditions, the optical receiver captures a strong solar background signal which competes with the useful optical signal returned from the aimed object. As reported in U.S. Pat. No. 7,508,497, background light is typically the largest source of noise during daylight operation of laser rangefinders.
An effective approach to make laser rangefinders more immune to the presence of intense background light is mentioned for example in U.S. Pat. Nos. 5,241,315 and 7,741,618. The approach includes two basic steps. The first step is to keep the FOV of the optical receiver as narrow as possible since the amount of background light collected by an optical receiver is proportional to its FOV, as shown for example in Eq. (2.10) of R. W. Byren, Laser Rangefinders, Chapter 2 of The infrared and electro-optical systems handbook Vol. 6, Active electro-optical systems, (SPIE Press, Bellingham, Wash., 1993). In turn, the second step consists in inserting a narrowband optical filter in front of the objective lens of the optical receiver. Interference optical filters having a bandpass of less than 10 nm are currently used in laser instruments intended for outdoor use and, by themselves, these filters also call for severely restricting the FOV of the optical receiver. This restriction comes from the fact that the center wavelength or the filter bandpass drifts significantly as the incoming light is captured at incidence angles way off the receiver optical axis. In addition, undue blocking of the useful return optical signal is prevented by ensuring that the wavelength spectrum of the emitted laser pulses remains narrower than the bandpass of the interference filter while its center wavelength coincides with that of the filter bandpass.
The basic configuration of optical rangefinder instruments has been subjected to various adaptations for use in applications related to vehicle safety and transportations. Nowadays, active optical sensing and rangefinder instruments are available for safety and driver assistance systems intended for adaptive cruise control (ACC), collision avoidance, pre-crash mitigation, blind spot detection, and parking assistance, just to name a few. Although these applications have their own constraints and requirements, they all share some common requirements. For example, these applications call for real-time, reliable detection and ranging of remote objects having a varying reflectance and located at distances of a few meters up to about 100 m. Furthermore, as discussed for example in U.S. Pat. No. 6,377,167, these applications require instruments capable of operating under a wide range of ambient lighting conditions, spanning from pitch-dark nighttime to bright sunlight when embarked in a vehicle. The present car safety applications also require instruments capable of optically sensing over a wide FOV, which can reach several tens of degrees along the horizontal direction. It is clear that this latter requirement cannot be fulfilled by directly integrating, without major modifications, the narrow-FOV optical rangefinders discussed in the preceding paragraphs.
A number of strategies have been envisioned for adapting optical rangefinder instruments for use in the various car safety applications mentioned above. For example, a wide FOV along the horizontal direction can be swept in a periodic fashion by mounting an otherwise standard optical rangefinder (having a narrow FOV) on a mechanical single-axis scanner device. This popular, yet simple approach provides large angular coverage while making an efficient use of the laser light radiated by the instrument. Apparatuses and methods relying on scanning devices and intended for various vehicle safety applications are taught for example in U.S. Pat. Nos. 5,249,157, 5,293,162, 5,604,580, 5,625,447, 5,754,099, 5,757,501, 6,317,202, 6,937,375 and 7,187,445. Unfortunately, this approach has some major pitfalls. Indeed, it is based on mechanical hardware that is expensive and often too bulky for widespread use in most car vehicles since it requires a rugged, long-life angular scanner assembly capable of withstanding the shock and vibration levels encountered during normal use of a car vehicle. Also, the narrow vertical FOV of typical scanner has poor performance for example when some sections of the surface of object are specular.
As reported in U.S. Pat. No. 7,532,311, the problems related to the use of mechanical scanner devices can be alleviated by developing non-mechanical scanning (NMS) techniques. These techniques include micro-electro-mechanical system (MEMS) devices, liquid crystals, and acousto-optical devices. Unfortunately, the development of NMS-based devices affordable enough for integration in car vehicles is still at its infant stage and faces serious difficulties related to the fact that both the emitted laser light and the optical axis of the optical receiver must be scanned together in perfect synchronism.
In addition to the techniques based on mechanical scanning devices, some other approaches allow for optical sensing with a wide angular coverage while not requiring any scanning. A first approach consists in combining a light emitter designed for illuminating over a large angular extent (also referred to as a Field Of Illumination (FOI)) with an optical receiver having a wide FOV. Both the light emitter and the optical receiver point in a specific, well chosen common direction (line of sight) determined by the nature of the intended application. The center position of the field of illumination is for example made coincident with the center position of the FOV of the optical receiver in such a manner that an object located anywhere within the FOV can be illuminated by a part of the emitted light. In most cases, the outer peripheries of the field of illumination and of the FOV would have their longest extensions along the horizontal direction, while remaining relatively narrow along the vertical. Although laser sources can certainly be used for flooding light over a wide field of illumination, this choice may be found to be expensive since the spreading of the laser light over a large angular extent calls for laser sources capable of emitting light pulses carrying high optical energy or, equivalently, high peak power. Stacked laser diode arrays fabricated by stacking a number of laser bars in a common substrate are for example well suited for providing high peak laser power over a wide field of illumination, but they still remain too costly for widespread use in car vehicles.
The FOV of the optical receiver can be widened by placing a photodetector with a larger photosensitive surface at the focal plane of an objective lens having a short effective focal length. Photosensitive surfaces with an elongated, rectangular shape are preferred for sensing over a FOV that extends along a given direction while remaining very narrow along the orthogonal direction. Although the use of a single photodetector impacts favorably on the costs of optical sensing instruments, it cannot really be envisioned in most applications because no angular resolution is provided within the FOV. Indeed, in addition to ranging objects present within the FOV, most applications call for determining, at least approximately, the angular positions of the objects relative to a reference direction Likewise, the ability to evaluate the approximate projected size and shape of the ranged objects provides a further key advantage for many applications by allowing classification of the detected objects, i.e., determining if they are cars, sport utility vehicles, heavy trucks, motorcycles, bicycles, pedestrians, masts, environmental particles, pavements, walls, etc.
A simple way of enabling angularly-resolved optical ranging of objects within a full FOV of sizeable extent is to split the full FOV into a set of contiguous, smaller FOVs through the use of multiple photodetectors disposed side-by-side in the focal plane of an objective lens. Each individual photodetector then has its own FOV, which, in this case, is generally referred to as the Instantaneous FOV (shortened hereinafter as IFOV), with the center angular position of any given IFOV determined by the transverse spacing between the center position of the corresponding photosensitive surface and the optical axis of the objective lens. The photodetectors just need to be disposed along a line to enable optical sensing over a whole FOV that spreads along a single direction. Linear arrays of photodetectors are then preferred as sensing devices in these scannerless, multiple-FOV optical sensing configurations.
As an illustrative example, U.S. Pat. No. 4,634,272 teaches an exemplary embodiment of such an optical sensing instrument for ranging objects in front of a vehicle. An array of three photodetectors is disclosed, with a mention that the number of photoelements can be increased for determination of the sizes and shapes of the objects to be sensed. In practice, an instrument based on the invention taught in the above-cited patent would require very sensitive photoelements because the objects within the FOV are ranged using the standard, analog method wherein an electronic counter is stopped as soon as a first pulse return is detected. As noted previously, highly-sensitive photoelements such as APDs are difficult to operate in outdoor settings due to the intense parasitic background light that is captured during daytime, for example when sensing over wide FOVs. Furthermore, this analog method does not allow discrimination (separate detection) of more than a single object that could be present within the IFOV of any given photoelement when these objects would be at different ranges.
Another example is given by U.S. Pat. No. 6,404,506 to Cheng et al., which teaches a non-intrusive laser ranging instrument comprising a 25-element linear array of APDs for detecting objects moving across a planar surface. The instrument is placed above a portion of a road and it is intended for traffic monitoring applications by measuring the travel time of vehicles that pass through the FOV of the instrument.
As compared to optical sensing instruments relying on scanning devices, the scannerless multiple-FOV instruments briefly described herein lend themselves to highly-integrated optical sensing solutions for automotive applications, owing to the use of very compact and robust linear arrays of photodetectors. The full set of IFOVs can be sensed in parallel, in a simultaneous fashion, by coupling the outputs of the photodetector linear array to suitable multi-channel amplifying electronics and signal/data processing means. The scannerless, multiple-FOV optical sensing instruments provide timely detection and ranging of objects entering in the full FOV of the instrument while classification of the detected objects is made possible by using a linear photodetector array made up of a large number of photoelements with small photosensitive surfaces. Linear arrays comprising various numbers of highly-sensitive APDs are currently available on the market, but these components get very expensive for integration in car vehicles as the number of photoelements increases. Using a linear array with a lower number of photoelements, each having a wider photosensitive surface, is more cost-efficient, but APDs with larger surfaces are still expensive. More importantly, the operation of APDs of larger diameter is plagued by a stronger capture of background light, and this often rules out their integration in systems intended for use in widely-varying ambient lighting conditions. The optical suppression of a large part of the extraneous background light with the help of a narrow bandpass interference filter does not work in a satisfactory manner in systems that must sense over wide FOVs.
There is a need, in the industry, for a scannerless, multiple-field-of-view optical rangefinder instrument adapted for low-cost integration and that can operate under a wide range of ambient lighting conditions, and for example under bright sunlight.